Emerging data indicate that an HIV vaccine could protect by inducing sufficient titers of broadly-neutralizing antibodies that mediate both direct viral neutralization as well as Fc-dependent effector functions against the diverse strains of HIV. Recent antibody discovery efforts have revealed a large number of new broadlyneutralizing monoclonal antibodies (bnMAbs) with extraordinary potency and breadth, raising the prospect that protective titers could be practically achieved if similar bnMAb specificities could be induced. vStructural studies of these new bnMAbs have revealed that they utilize structural and genetic features that appear to be unique to humans. We therefore propose to employ the humanized BLT mouse model to evaluate and optimize vaccines targeting new highly potent bnMAbs. BLT mice possess human humoral, cellular, and innate repertoires. We have demonstrated that the human immune system in these mice supports sustained HIV infection and generates human class-switched neutralizing antibodies in response. In this project, we propose to take a three-pronged approach to optimize vaccines targeting these new bnMAbs in the BLT mouse model, in Aim 1, we will develop a more complete understanding of the human antibody response to HIV infection generated in BLT mice, in order to utililize this model system most effectively for vaccine research, including for HIV vaccine challenge studies. In Aim 2, we will develop a more complete understanding of the ability of bnMAbs to protect against HIV infection following passive transfer to BLT mice in vivo, comparing b12, for which protection has been documented in the established macaque SHIV challenge model, with the new more potent and broad bnMAbs targeted by our current vaccine design efforts. Using selectively mutated antibodies, we will also compare the relative protective contributions of the neutralizing activities of these antibodies to their Fc receptor-dependent effector functions. In Aim 3, we will employ BLT mice to evaluate and optimize novel epitope-focused immunogens and immunization regimens designed to induce potent and broad bnMAbs like VRC01, targeting the CD4-binding site, and 10E8, targeting the membrane proximal external region.